Three-phase voltage source inverters (VSI's) are generally used to convert DC power into three-phase AC power. Typically, the three-phase output voltages are sinusoidal waveforms spaced 120 degrees apart, to be compatible with a wide variety of applications requiring conventional AC power. In general, the output power frequencies commonly used are 50, 60, and 400 hertz, but other frequencies could be used as well. One current example of an inverter application is the electric or hybrid automobile, where a DC power source, such as a battery, fuel cell array, or other equivalent device, is converted into an AC power supply for various internal control functions, including the propulsion system.
The quality of an inverter is generally determined by its output voltage and frequency stability, and by the total harmonic distortion of its output waveforms. In addition, a high quality inverter should maintain its output stability in the presence of load current variations and load imbalances.
In the case of unbalanced loads, the 4-leg three-phase inverter topology is generally considered to offer superior performance than a 3-leg three-phase topology. That is, with an unbalanced load, the three-phase output currents from an inverter will generally not add up to zero, as they would in a 3-leg balanced load situation. Therefore, a fourth (neutral) leg is typically added to accommodate the imbalance in current flow caused by an unbalanced load. If a neutral is not used with an unbalanced load, voltage imbalances may occur at the load terminals, and the output power quality may be adversely affected.
The operational functions of a typical inverter are generally controlled by drive signals from an automatic controller. The controller and inverter are usually implemented as a closed loop control system, with the inverter output being sampled to provide regulating feedback signals to the controller. The feedback signals typically include samples of the output voltage and current signals, and can also include harmonics of the fundamental output frequency.
The ability of an inverter control system to compensate for undesirable harmonics is generally limited by the bandwidth of the system voltage control loop, which may not be adequate for compensating high frequency harmonic distortion. For example, in a typical cascaded voltage/current regulator configuration, the voltage loop bandwidth is generally limited to approximately 1/100th of the sampling frequency. Due to technical factors, the sampling frequency is usually limited to a range of 5 to 20 kHz, thus limiting the voltage loop bandwidth to a range of 50 to 200 Hz. Therefore, harmonic compensation and transient response would be limited to frequencies within this range.
Moreover, the transient response characteristics of an inverter control system may also be limited by the overall execution time of the regulating loop software modules. That is, the larger the number of software modules, the greater the execution time, and the slower the transient response.
Accordingly, it is desirable to provide an inverter controller with a relatively high voltage control loop bandwidth, for improved harmonic compensation and transient response. In addition, it is desirable to provide an inverter controller with a minimal quantity of software modules, in order to speed up execution and reduce throughput time. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.